Genes are regulated in an inducible, cell type-specific or constitutive manner. There are different types of structural elements which are involved in the regulation of gene expression. Cis-acting elements, located in the proximity of, or within genes, serve to bind sequence-specific DNA binding proteins, i.e., trans-acting factors. The binding of proteins to DNA is responsible for the initiation, maintenance, or down-regulation of gene transcription.
Cis-acting elements which control genes include promoters, enhancers and silencers. Promoters are positioned next to the transcription start site and function in an orientation-dependent manner, while enhancer and silencer elements, which modulate the activity of promoters, may be flexible with respect to their orientation and distance from the transcription start site.
An example of a specifically regulated gene in plants is phenylalanine ammonia-lyase (PAL), which catalyzes the deamination of phenylalanine to cinnamic acid, the precursor of a wide variety of natural products based on the phenylpropane skeleton. During vascular development, PAL is selectively expressed in differentiating xylem cells associated with deposition of the structural polymer lignin. Lignin, the second most abundant biopolymer after cellulose, is the major structural cell wall component of cells forming vessels in plant tissue (xylem). The xylem is responsible for movement of water and inorganic solutes from plant roots to plant shoots. PAL genes are expressed at correspondingly high levels in differentiating xylem.
The ability to artificially regulate the rate of gene expression provides a means of producing plants with new characteristics. There are numerous situations in which increased levels of gene expression, including increased endogenous gene expression, may be desirable. Such situations include, for example, production of protein plant products for agricultural or commercial purposes.